First, it conserves their energy. Each bird flies slightly above the bird in front of him, resulting in a reduction of wind resistance. The birds take turns being in the front, falling back when they get tired. In fact, a flock of geese can fly 70 percent farther by adopting the V shape rather than flying in isolation.
The second benefit to the V formation is that each bird has an unobstructed field of vision, allowing flock members to see each other and communicate while in flight. Fighter pilots often use this formation for the same reason.